The present invention relates to a multivibrator and, more particularly, to a multivibrator which is useful for a pulse count type FM demodulation circuit.
The well known pulse count type FM demodulation circuit operates by producing pulses each having a fixed width and being synchronized in phase with each period of the signal to be demodulated, and by integrating the pulses. The fixed width pulses may be produced by first wave-shaping the signal to be demodulated in an amplitude limiter and then applying that wave-shaped signal to a differentiating circuit which converts it into negative or positive going pulse signals. Use of pulse signals obtained in this manner, however, results in poor detection efficiency and thus, a poor signal to noise ratio. To overcome such disadvantages, a monostable multivibrator may be used to expand the pulse width. In superheterodyne receivers, it is necessary to carefully tune the received signal so that it will be centered in the frequency band of the band-pass filter used in the intermediate frequency amplifying stage to most effectively minimize signal distortion. Most receivers of this type are provided with tuning meters to permit easy selection of the appropriate tuning point. A DC output signal from the FM demodulation circuit is typically used to drive the tuning meter to indicate the signal tuning point. When the DC output signal of the demodulation circuit drifts due, for example, to a change in the power source voltage or the temperature, the tuning meter incorrectly indicates the tuning point. The pulse count type FM demodulation circuit is capable of excellent linearity but conventional monostable multivibrators used in such circuits are very sensitive to temperature variations. Accordingly, such DC output signals from the circuits may change with the temperature variation, thus providing incorrect signal tuning indications, resulting in incorrect tuning. When this happens, there is adverse signal distortion.